Syphilis, also called Lues, is a severe infectious disease which is caused by Treponema pallidum, belonging to the bacterial family of spirochetes. It is mainly transmitted by sexual contact but can also be passed from an expectant mother to the unborn during pregnancy. The disease is characterized by distinct clinical stages and long periods of latent, asymptomatic infection. Many infected individuals do not notice symptoms and thus are unaware of their syphilis infection for years. The primary infection is confined and usually causes a small painless ulcer (primary stage, “Lues I”). If left untreated by penicillin, the disease proceeds to the secondary stage Lues II (about eight weeks after infection), which entails flu-like symptoms, non-itchy skin rash and swollen lymph nodes. After some years, at stage Lues III, syphilitic nodes appear throughout the body. The final stage (Lues IV) is characterized by destruction of the central nervous system eventually leading to neurological and cardiological disorders, general paralysis, ataxia, dementia and blindness.
Although effective therapies have been available since the introduction of penicillin in the mid-20th century, syphilis still remains an important global health problem with estimated 12 million new infections worldwide each year. It is necessary to reliably identify patients with Treponema infection in order to initiate antibiotic therapy and thus to prevent the further spread of syphilis. As a consequence, it is necessary to provide reliable diagnostic tools such as immunoassays for the detection of antibodies against Treponema pallidum. Yet, in order to be used as specific compounds in serological applications, recombinant-derived proteins have to meet several requirements such as solubility, stability and antigenicity.
TpN17 (Treponema pallidum strain Nichols, 17 kDa), a small protein that consists of 134 amino acid residues in its mature form, is the immunodominant antigen of Treponema pallidum, the causative agent of Syphilis (J. Clin. Lab. Immunol. (1998), 50, 27-44; Folia Microbial. (2003) 48 (4), 549-553). Antibodies towards TpN17 are frequent and abundant in Treponema-infected individuals, and thus it is advantageous to use TpN17 in certain embodiments of an immunoassay that aims at the sensitive and reliable detection of Treponema infections.
However, we observed that an immunoassay using TpN17 as an antigen tends to show false positive results, i.e. it provides a seemingly positive signal although in fact no antibodies against Treponema are present in that sample. These interferences are a rare but significant phenomenon. They compromise the specificity of the immunoassay and they are clearly due to the use of the Treponema pallidum antigen TpN17, which is virtually indispensable in a Syphilis immunoassay.
The problem underlying the current invention can therefore be seen in providing means and methods for avoiding false positive results and increasing the specificity of TpN17-based immunoassays for the detection of anti-Treponema antibodies.